The present invention relates generally to a continuously variable transmission (CVT), and more particularly to the surfaces on a pulley of the CVT.
Some continuously variable transmissions (CVT) use pulley assemblies with one truncated conical member that is movable relative to another truncated conical member, with a belt or chain mounted between them. The ratio of the input pulley to the output pulley is adjusted by varying the spacing between the conical members. The torque is transferred via friction between the conical members and the belt or chain.
In vehicles having engines with higher power output, the CVT needs to have a higher torque capacity. The higher torque capacity is achieved, in part, by maintaining a high frictional engagement between the belt/chain and the conical members. This must be achieved, however, while still providing adequate wear resistance of the conical surfaces to assure long term durability of the transmission. As the CVT is operated, the belt incurs micro-slippage due to torque transfer and the wrap angle difference between the primary and secondary pulleys. Over time, the micro-slippage causes a surface texture depth reduction due to mixed boundary lubrication conditions, which, with usage, may change the coefficient of friction between the belt and the pulley surface.
Pulleys with high surface roughness tend to have higher friction carrying capability with minimal slippage. As a result, the conical surfaces are treated to have a high average roughness (Ra). Some methods for creating this surface roughness may have included shot peening, grinding with a grinding wheel, stone polishing or tape polishing. While these mechanical methods of creating surface roughness produce a desired average, the roughness is random and not consistent, so the roughness just meets an overall average. The actual peak to valley height that determines the average roughness is not consistent throughout the treated surface area, thus a precise, engineered surface is not produced.